These working liquids should satisfy four basic requirements: a low proper vapor tension, a high thermo-oxidation stability, relatively low viscosity at a temperature of 10.degree.-20.degree. C. and a high ultimate vacuum produced, namely 2-5.times.10.sup.-8 mm Hg in unbaked systems without nitrogen traps and 2-5.times.10.sup.-9 mm Hg in baked systems without nitrogen traps.
Known in the art are a number of working liquids possessing the above-mentioned properties. The best among them are allmeta-pentaphenyl ether (Convalex) and silicon oils.
These oils, however, feature a series of essential disadvantages. Thus, allmeta-pentaphenyl ether crystallizes at a temperature of 38.degree. C. It is toxic and forms, after long-term operation, solid difficultly removable scaling on the inner surfaces of pumps and piping, thus impairing performance of vacuum units.
A common disadvantage of the prior art oils resides in the complicated process for their manufacture, and, correspondingly, a high production cost.
Known in the art is a process for producing a high-vacuum oil by way of alkylation of naphthalene with individual higher .alpha.-olefins containing 20 to 24 carbon atoms per molecule. The process is conducted at an elevated temperature in the presence of acid-type clay as a catalyst. These monoalkylnaphthalenes provide for a super-high vacuum when used in diffusion pumps of about 2.8-4.8.times.10.sup.-9 mm Hg in heated systems (cf. U.S. Pat. No. 3,563,073; Cl.C 07 C 15/24; patented 1971).
However, difficulties associated with the production of individual higher alpha-olefins does not enable wide commercial implementation of this process and, consequently, a super-high-vacuum oil produced by alkylation of napthalene with individual higher alpha-olefins is substantially unavailable for industrial applications.
Furthermore, upon alkylation of naphthalene with olefins, oligomerization of the latter occurs at a considerable rate thus causing contamination of the desired product with dimers and trimers of olefins and, hence, strongly impairing the oils performance.
Also known in the art is a process for the production of a high-vacuum oil by way of alkylation of naphthalene with olefins formed upon dehydration of higher alcohols produced by hydrogenation of sperm oil. Vacuum oil is produced by alkylation of naphthalene with said olefins at a temperature within the range of from 20.degree. to 100.degree. C. in the presence of a complex catalyst 2BF.sub.3.H.sub.3 PO.sub.4. The alkylation product comprises a mixture of higher monoalkylnaphthalenes which after purification ensure a vacuum of 2.10.sup.-9 mm Hg in diffusion pumps of heated systems. (cf. USSR Inventor's Certificate No. 544,644, cl. C07c.sup.3, published 1977). Nevertheless, due to a sharp decrease of sperm whales this source of the starting raw material has become inaccessible and the above-described high-vacuum oil is not available in large-scale commercial production.
It should be also noted that, though oligomerization of olefins on the catalyst 2BF.sub.3.H.sub.3 PO.sub.4 occurs to substantially lower extent than with other catalysts, the desired product is still contaminated with oligomers and its purification therefrom under commercial production conditions is associated with substantial difficulties.